In general, an optical pickup system, as shown in FIG. 1, for compact disc players, video disc players, optical disc drivers and multi-disc players includes a laser diode 1 as a light source, a diffraction grating 2 generating tracking beams by dividing the laser beam emitted from the laser diode 1 into three diffraction beams (an 0 order 1 order -1 order), a collimator 3 converting each of the three diffraction beams from the diffraction grating 2 into parallel beams, an objective lens 5 focusing the three parallel beams from the collimator 3 onto an optical disc 4, a two division hologram 6 positioned between the diffraction grating 2 and the collimator 3 for diffracting the reflected beams of the optical disc 4 passed through the objective lens 5 and the collimator 3 successively, and a five division photodetector 7 the reflected beams diffracted by the two division hologram 6 being focused thereon.
The two division hologram 6 has a first hologram 6A and a second hologram 6B, and the photodetector 7 has a first to a fifth light reception area PD1 to PD5.
The laser beam emitted from the laser diode 1, passing through the diffraction grating 2 for generating tracking beams, is divided into 0 order, 1 order and -1 order diffraction beams. The 0 order laser beam serves to detect focus errors and information signals of the optical disc 4, and the 1 order and the -1 order laser beams serves to detect tracking errors.
Each of the three diffraction beams divided in the diffraction grating 2, after passing the hologram 6, is convened into parallel beams in the collimator 3, which parallel beams are focused onto the optical disc 4 by the objective lens 5. The three laser beams focused on the optical disc 4 are reflected to pass the objective lens 5 and the collimator 3 successively and diffracted in different angles by the first hologram 6A and the second hologram 6B of the two division hologram 6 to be focused onto the first through the fifth light reception area PD1 thru PD5.
The 0 order laser beam is focused onto the fourth light reception area PD4 by the first hologram 6A and onto a boundary line between the second and the third light reception area PD2 and PD3 by the second hologram 6B. The one order and the minus one order laser beams are focused onto the first and the fifth light reception areas PD1 and PD5 by the first and the second holograms 6A and 6B, respectively.
FIGS. 2(A) to 2(C) explains a change of beam shapes on the photodetector 7 according to a displacement of the optical disc 4 shown in FIG. 1.
Where a focus error signal, a tracking error signal and an optical information signal are defined as follows;
focus error signal=S2-S3 PA1 tracking error signal=S1-S5, and PA1 light information signal=S2+S3+S5,
wherein the S1 to S5 are the light reception signals of the first to the fifth light reception areas PD1 to PD5 of the photodetector 7, respectively.
It can be seen that, when the focus error signal is greater than zero, it indicates that a distance between the optical disc 4 and the objective lens becomes wider (FIG. 2(B)), when less than zero, becomes narrower (FIG. 2(C)) and when zero, no focus error has been occurred (FIG. 2(A)). Also it can been seen that, according to the tracking error being &gt;0 or &lt;0, whether the main beam (zero order diffraction beam) follows along a track of the optical disc 4 exactly. By means of such focus error signal and tracking error signal, the focus error and the tracking error can be corrected enabling to read an optical information correction signal recorded on the optical disc 5 correctly.
In the meantime, the wave length of the laser diode 1 used as a light source can be changed depending on environmental temperature wherein, when the wave length is changed, because of the diffraction angle changes by the diffraction grating 2 and the two division hologram 6, an error similar to the focus error and the tracking error develops. As shown in FIG. 3 which is an enlargement of "A" part shown in FIG. 2(A), when the boundary between the second and the third light reception areas PD2 and PD3 of the photodetector 7 is kept to have a certain angle .theta., the error by the change of the wave length can be corrected.
For example, when the wave length of the laser beam emitted from the laser diode 1 is 780 nm, the laser beam focuses onto a first position P1, when 775 nm, onto a second position P2, and when 785 nm, onto a third position P3. Because the focus error signal can be kept to be zero even though the wave length of the laser beam is changed in a state when the focus error has not been developed when a line connecting centers of the focusing positions P1 to P3 moving as described above is taken as the boundary line between the second and the third areas PD2 and PD3, an error according to the change of the wave length can be prevented.
However, in a optical pickup system described above, as a three beam detection method, a separate diffraction grating which divides a laser beam into three beans to be used. Accordingly alignments between the diffraction grating and the hologram and between the diffraction grating and the hologram, and the photodetector is required, which alignment technique is very difficult resulting to raise a problem of dropping the productivity. Further, prevention of errors come from changes of wave length by sloping the boundary line between the second and the third light reception areas of the photodetector for correction of the wave length of the laser beam, drops productivity making the production cost hike.